There are three ways to get a client into a flow.
From the Flows page
With the flow open, click Add contacts in the top right.
Choose the Stage they'll start in.
Search for and select one or more clients — use Select all or Clear to manage your selection quickly. Clients already in this flow won't show up here.
Click Add to flow.
From a client's profile
You can also add a client to a flow directly from their profile, using the profile's Actions menu.
From the Reporting page
Contacts can also be added to a flow from the Reporting page.